Field of the Invention
The present invention relates to an annular combustion chamber for a turbine engine such as an airplane turbojet or turboprop.
Description of the Related Art
In known manner, an annular combustion chamber for a turbine engine receives upstream a stream of air from a high pressure compressor, and delivers downstream a stream of hot gas for driving the rotors of high-pressure and low-pressure turbines.
An annular combustion chamber comprises two coaxial walls forming surfaces of revolution extending one inside the other and connected together at their upstream ends by an annular chamber end wall, the chamber end wall having openings for mounting fuel injection systems between the inner and outer coaxial walls.
Each injection system includes means for supporting the head of a fuel injector and at least one swirler that is arranged downstream from the head of the injector, on the same axis, and that delivers a rotating stream of air downstream from the injection of fuel so as to form a mixture of air and of fuel that is to be burnt in the combustion chamber.
The swirlers of injection systems are fed with air coming from an annular diffuser mounted at the outlet from the high-pressure compressor arranged upstream from the combustion chamber.
Each swirler leads downstream to the inside of a mixer bowl having a substantially frustoconical downstream wall that flares downstream and that includes a row of air injection orifices that are regularly distributed around the axis of the bowl.
The outer coaxial wall of the combustion chamber has an annular row of primary dilution orifices and at least one spark plug leading to the inside of the combustion chamber and arranged downstream from the primary dilution orifices.
In operation, air leaving the high-pressure compressor flows inside each of the injection systems. The air/fuel mixture is ejected from each injection system so as to form a sheet of air and of fuel that is substantially frustoconical, flaring downstream. The aperture angle of the sheet is a function of the aperture angle of the frustoconical wall of the mixer bowl and of the dimensions of the air injection orifices formed in said frustoconical wall. Thus, the greater the diameter of the orifices in the mixer wall, the greater the flow rate of air passing through each of the orifices, and the less the extent to which the sheet of air/fuel mixture flares.
The primary dilution orifices serve to stabilize the combustion flame in the end of the chamber, and by diluting the air/fuel mixture they prevent the combustion flame from separating and penetrating into the high pressure turbine and damaging components, such as specifically stator vanes, by forming hot points thereon.
In practice, injection systems are configured so that for each injection system, the air/fuel mixture sheet crosses or intersects circumferentially the fuel sheets of the two adjacent injection systems, and does so upstream from the dilution orifices. This ensures circumferential continuity of the air/fuel mixture between the injection systems prior to dilution, thereby serving to guarantee that the flame ignited by the spark plug(s) propagates all around the circumference of the combustion chamber.
In certain configurations, in particular in so-called converging combustion chambers in which the outer and inner coaxial walls are frustoconical walls of section that taper downstream, or when the number of injection systems is small, the circumferential pitch between adjacent injection systems is greater. As a result the sheets of fuel from adjacent injection systems no longer intersect circumferentially upstream from the primary dilution orifices, thereby giving rise to difficulties in propagating the flame circumferentially between the injectors, and thus reducing the performance of the combustion chamber.
In order to mitigate that drawback, it is not desirable to increase the number of injectors, since that would lead to making the turbine engine heavier. Increasing the aperture angle of the sheets of fuel is also unsatisfactory, since that would lead to projecting a larger quantity of fuel towards the inner and outer coaxial walls and to forming hot points on the inner and outer coaxial walls.